ci: compare the schema the MODELS produce against the migrations (#3275)

baseline.yml only ever compared migrations against migrations. The
question #3275 exists because nobody had ever asked the other one: does
a database built from the MODELS match the one the chain produces?

`mode: models` answers it. It applies the candidate autogenerated from
the models instead of this tree's revisions, and diffs that against the
chain. A clean run means --autogenerate is trustworthy again, which it
demonstrably has not been: against the pre-reconciliation models it
would have proposed dropping eleven indexes and two uniqueness
guarantees.

The two extensions are created by hand in that mode. They are database
objects rather than table metadata, so no model can carry them — their
absence is outside what this comparison asks about, and silently
tolerating it is correct rather than a filter that hides a defect.

Also declares the HNSW index on the ImageRecord model. SQLAlchemy can
express an hnsw access method with an operator class
(postgresql_using + postgresql_ops), so there was never a reason for it
to live only in 0036. That removes the last item from the list of things
a generated baseline cannot reproduce, leaving only the two extensions.

backend/app/models/image_record.py

@@ -33,6 +33,17 @@ class ImageRecord(Base):
 
 
     __table_args__ = (
+        # alembic 0036, and the last thing in this schema that lived only in a
+        # migration. SQLAlchemy CAN express an hnsw index with an operator
+        # class, so there is no reason for it to be invisible to the models —
+        # and its absence was the quietest failure of the lot: everything
+        # works, similarity search just silently stops using an index.
+        Index(
+            "ix_image_record_siglip_hnsw",
+            "siglip_embedding",
+            postgresql_using="hnsw",
+            postgresql_ops={"siglip_embedding": "vector_cosine_ops"},
+        ),
         # alembic 0035/0071: the date-ordered browse indexes (#3275).
         Index("ix_image_record_effective_date", text("effective_date DESC"), text("id DESC")),
         Index("ix_image_record_earliest_post_date", text("earliest_post_date DESC"), text("id DESC")),